Regarding an optical network, such as a fiber to the home (FTTH), a cable television (CATV), and the like, an optical transmission system of a passive double star (PDS) type is used, in which a passive device is provided for a branching part branching a cable into the number of users on the way of a path from a central office to the users. Regarding the passive device, there is provided such as a splitter for example.
Such the splitter is to be connected with an optical fiber to be taken out for the user side from each optical subscriber unit (OSU) provided in an optical line terminal (OLT) at the central office. Moreover, optical paths branched by the splitter are to be connected with optical network units (ONU) for a plurality of users, via a plurality of optical fibers.
According to such the optical transmission system of the PDS type in which the OLT and a plurality of ONUs are connected using the optical fibers and the splitter, data are to be transmitted using an optical signal of only one wavelength. Hence, it is had for improving a large capacity.
On the contrary, a system for being able to perform a wavelength division multiplexing (WDM) communication, with using the ONU in such the existing optical transmission system without changing any, is reported in the below described patent document 1.
Regarding the system, as shown in FIG. 12, for optical fibers 104 connecting between an OSU 102 in an OLT 101 and a splitter 103, a first multiplexer/de-multiplexer (MUX/DEMUX) 105, an optical transmission path 106, a second MUX/DEMUX 107, and then a wavelength converter 108 are to be connected in order therewith, as a configuration thereof. Thus, according to the configuration, the existing splitter 103 and the existing ONU 109 are become able to be used. Moreover, between the OLT 101 and the ONU 109, a sufficient allowable dissipation becomes to be given, and then a distance restriction from dissipation becomes able to be removed. Therefore, it is reported therein that the large capacity and a long distance communication becomes able to be improved.
Regarding the wavelength converter 108, as shown in FIG. 13, a common configuration for wavelength conversion is to be adopted.
The common wavelength converter comprises two of 3 (dB) couplers 110a and 110b to be connected with the second MUX/DEMUX 107 and the splitter 103 respectively. Moreover, for both up and down as two pathways signalling system between the two of 3 (dB) couplers 110a and 110b, it comprises opto-electric conversion devices 111 and 121 as light receiving elements, 2R/3R receiver circuits 112 and 122, driver circuits 113 and 123 for the light receiving elements, light emission elements 114 and 124, and isolators 115 and 125, to be connected in order toward the signal propagation direction respectively. Thus, there is provided a configuration thereby.
Moreover, such as disclosed in the patent document 1, there is reported a configuration regarding the wavelength converter 108, with omitting the light receiving element 121, the receiver circuit 122, the driver circuit 123, and the light emission element 124 on the down signalling pathway.
Furthermore, such as in a below disclosed nonpatent document 1, there is reported a configuration that, for a down signalling system of the optical signalling pathways between the OLT and the splitter, a 2.488 Gbit/s converter, a dense wavelength division multiplexer (DWDM), and another converter are connected in order toward the signal propagation direction therein. Moreover, for a up signalling system therein, a converter, a DWDM, and an optical channel filter are connected in order toward the signal traveling direction, as the configuration. And then the long distance between the OLT and the ONU becomes to be improved. Moreover, the DWDM is to be comprised of a semiconductor laser, a modulator, an optical receiver, a variable optical attenuator (VOA) and the MUX/DEMUX.
[Patent Document 1] Japanese Patent Application Publication No. 2002-261697
[Nonpatent Document 1] R. P. Davey et al. “DWDM reach extension of a GPON to 135 km” PDP35, 2005, Optical Society of America